Flow cytometry was first used to isolate a stem cell when the laboratory of Dr. Irving Weissman reported the prospective identification of mouse hematopoietic stem cells in 1986. In 2008, flow cytometry still sets the standard for cell enrichment. Thus, a state of the art flow cytometry core is critical for the isolation of solid tumor and leukemia stem cells for the 3 projects of this proposal. When working with human specimens obtained from the pathologist, one cannot predict with certainty when a specimen will arrive. This is predominantly because of the inherent uncertainty of surgical scheduling and issues that arise during an operation. The successful isolation of cancer stem cells is dependent upon the speed in which a tumor or blood sample is processed and sorted. Thus, one must be able to access a flow cytometer quickly for studies involving human tumor cells. This ensures that each tumor sample is analyzed in a timely and efficient manner, and data from each precious sample is not compromised by delays in processing because of lack of access to a flow cytometer. This is one of many reasons why the Stanford University Stem Cell Institute has established a flow cytometry core for the use of investigators in the Institute. Although commercial flow cytometers are quite sophisticated, they need to be modified for optimal isolation of cells from different tissues. In addition, each of the flow cytometers in this core has been modified so that both hematopoeitic and solid tissue stem cells can be isolated using optimal conditions. This is made possible in part by the core leader, David Parks. Based on Institute user's needs, the flow cytometers receive customized factory components. In addition, factory settings have been modified by the core PI to optimize sorting efficiency of both blood and solid tissue stem cells.